How to avoid demotivation while working on a project?

One way to think of it is that motivation is something that will cycle up and down based on a lot of factors.

So,

1. Prepare a bunch of tasks that are super-easy and can be done during low motivation times. The best time to do this prep is when your motivation is high.
2. Take a break if you need it. Eat something. Drink water -- you might just be tired, hungry, or dehydrated.

keep a list of 2 to 5 things that need to be done, with a clear end goal. work on all those projects simultaneously (get annoyed with one move to the other), that way you are always seeing the fruits of your labor.

Find out what is demotivating you.
Address that situation.